Melting and purification of uranium



Sept. 16, 1958 F. H. SPEDDING ETAI. 2,852,364

MELTING AND PURIFICATION 0F URANIUM Filed Sept. 20, 1944 2 Sheets-Sheet l Sept. 16, 1958 F. H. sPEDDlNG ETAL 2,852,354

MELTING AND PURIFICATION oF URANIUM 2 Sheets-Sheet 2 Filed ept. 2o, 1944 0 5 i w J\ f 5 r j v Y \\l\\\ I@ W 2 m 7 7 few? F /f w 5mg@ United States Patent MELTING AND PURlFICATIGN F URANIUM Frank H. Spedding and C. F. Gray, Ames, Iowa, as-

signors to the United States of America as represented by the United States Atomic Energy Commission Application September 20, 1944, Serial No. 554,920

6 Claims. (Cl. 7584.1)

The invention relates to a casting process and apparatus for treating uranium metal.

Where uranium has been obtained in the form of substantially pure metal, the reactivity of the uranium causes impurities in the form of uranium compounds, such as uranium oxide, to be associated with the metal immediately upon exposure of the metal to ordinary atmospheric conditions. The formation of uranium oxide, for example, gives the uranium metal la hard, dense skin or film which tends to increase in thickness as the metal is continued to be exposed to air.

Uranium metal in the form of an ingot `produced by the reaction of a uranium halide and a reducing metal may include impurities such as uranium carbide, calcium oxide, or the like, that are solid at the melting and casting temperature of uranium. Such impurities are usually on the surface of the ingot and form a part of the skin of uranium oxide.

In the reaction, in which calcium, magnesium, or other reducing metal is reacted with a uranium halide, a portion of the reducing metal may be associated on or within the uranium metal mass. Such reducing metal, and particularly magnesium, is volatile at the melting and casting temperatures of uranium and may be removed in vapor form during the casting operation.

It has been found that uranium metal may be separated from the solid impurities such as the skin portion of the ingot by heating a uranium mass containing such impurities to `at least the melting point of the uranium metal and then withdrawing the molten uranium away from the solid impurities. Thus it has been found that the skin portion of a uranium body is of higher melting point than the interior portion and thus it is possible to heat the body to a temperature such that the interior becomes molten while the skin portionremains essentially solid. The separation may be carried out by supporting the uranium mass in such a manner that the uranium after being converted to a molten state will cause the skin or film of impurities surroundingfthe uranium metal to burst, with the molten uranium then owing away from the `solid impurities. Thus, the uranium mass is supported at spaced points in a suitable container and the uranium converted to a molten state. The separation is eifected by bursting the skin of impurities on the exterior of the mass by the weight `of the molten uranium bearing upon the unsupported points and flowing the molten uranium Vbeyond the supporting points while maintaining the impurities on the support.

The volatile impurities may =be removed, at leastin part, during the melting of the uranium ingot although such removal is preferably effected by maintaining the cast uranium at a temperature above the boiling point `of the volatile impurities for sucient time to vaporize such impurities and thus remove them from the uranium. This invention relates to improvements in the processes and apparatus disclosed in the copending application of George Meister, Serial No. 533,112, led April 28, 1944, now Patent No. 2,756,138,`issued July `24, 1956.

y, 2,852,364 ce Patented Sept. 16, 1958 It is an object of the invention to provide an improved process and apparatus for purifying uranium metalyfor separating uranium metal from Various impurities associated with it that are in solid form at the casting temperature of uranium; and for removing from the uranium impurities that are volatile at the melting and casting temperature of uranium. A further object of the invention is the provision of a process and apparatus for casting molten uranium metal into a carbon mold `without excessive formation -of uranium carbide. It isa still further object of the invention to obtain a shaped product of exceptionally pure uranium metal by casting molten uranium metal into `a carbon mold. Other objects and advantages will appear from the following description and drawings appended thereto.

ln the drawing, in which like reference characters refer to like parts, Fig. 1 is a vertical, sectional View 0f an apparatus for carrying out the invention; Fig. 2 is a view taken on line 2-2 of Fig. l; Fig. 3 is a view similar to Fig. 2 showing a modified form of the apparatus; Fig. 4 is an enlarged fragmentary sectional lview of a portion of the apparatus shown in Fig. 1; `and Fig. 5 is a fragmentary sectional view of a modied form of the apparatus.

In one preferred embodiment of the invention, an apparatus is provided as shown in Fig. 1 in which uranium ingots 1 are placed within a melting chamber 2 formed by crucilble 3 having a lid 4 yand a perforate bottom `5. The melting cham'ber is `of suiicient interior diameter to contain one or more uranium ingots having both skin and interior impurities, and is of suiicient wall thickness to resist the high temperatures and corrosion Vproducts of the casting operation. Lid 4 is designed -to engage the upper end :of Crucible 3 andpreferably has an orilice 7 to permit the escape of various volatile products given oit during the casting operation. To prevent deleterious effects of such volatile products, protective cap S may be placed over orifice 7. Cap 8 is suitably in the form of .an inverted cup with one or more side openings 9 to permit theescape of the volatile products. The cap prevents direct impingement of the volatile products upon outer elements of the apparatus. The lower portion of crucible 3 includes sloping side edges 10 which terminate at perforated bottom 5. Preferably, bottom 5 consists of a removable circular perforate plate which seats into rabbets 11, provided for that purpose.

Bottom 5 includes one or more perforations 12 as shown in Figs. 1 and 2, through which themolten uranium ilows during the casting operation. The size of the openings 12 are dependent somewhat upon the amount and area =of skin impurities that are associated with the uranium ingot 1. In practice, it has `been found that openings of a diameter ranging from JG to 1 are suitable for this purpose.

The crucible rests upon and is supported by mold 13 which contains one or more casting chambers 14. Chamber 14 is of the shape of the desired casting and should Fbe of a size suidcient to receive all of the pure uranium contained in ingots 1. Crucible 3 may be secured on mold 13 in alignment by a conventional shouldered joint.

The above described elements of the apparatus `mav be formed of any refractory material suitable for treatment of uranium. Beryllia, alundum, magnesia, sillimanite, or the like may be used although there is a tendency for such refractory materials to contaminate the cast uranium. Preferably, all of the described elements of the apparatus are formed of a hard, dense form of carbon such as graphite which may be machined to the proper size and shape. Although there is a tendency forthe molten uranium to react with the carbon to form `uranium carbide, for most purposes such carbide formation is preferred to contamination resulting from the use of other refractory materials for the apparatus; by proper casting procedure the uranium carbide formation is greatly minimized. A

The mold 13 rests upon insulating block 15 which may suitably be formed of -a refractory material such as firebrick.l For the purpose of obtaining a vacuum in the apparatus, channels16 may be formed in the insulating block'15. The insulator or block 15 is preferably supported by a water cooled brass head 17. Head 17 includes a water chamber 18 and inlet and outlet means 19. Channel 20 extends through the head in alignment with channel 16 of the insulating block. Projection 21 may be Yusedffoi-attaching suitable evacuating means to the head.Y

To permit the use of temperature measuring means such as thermocouples or the like, Vopening 22 is provided through the headwhich extends through the insulating member 15 and into mold 13. During the casting operation, thermocouples (preferably of the -chromel-alumel type) may be -inserted through opening 22 and into "sulation isin the form of sleeve or sleeves 23 and may be formed of thermo-setting sillimanite and sawdust which is shaped and then fired at a high temperature.

-Sleeves 2,3 may be laid up around the apparatus and a flat circular sheet 35 of the same material placed over the top, the sheet being protected by cap 8 from the deleterious effects of the volatile products given off during the casting operation. vThe insulating sleeves rest upon `an extension of head 17 provided for that purpose.

Inasmuch as the casting operation is preferablyperformed in a vacuum or in an inert atmosphere, shield -24 is used to fenvelop the entire apparatus, resting on flanges 25 of head 17. The shield is formed of a nonconducting resistant material such as quartz. To insure 'an airtight seal, gaskets 26 of rubber or the like are interposed between the end of the shield 24 and anges -25. -The shieldmay be further secured by sealing material 27 of ajhigh melting wax, pitch, or the like.

i To secure high` temperatures in melting chamber 2 and vcasting chamber 13, induction coil 28 is provided which is suitably arranged to surround shield 24 and to be raised and lowered into any desired position with respect to the apparatus. s

-The apparatus is assembled by placing insulating blocks 15 on head v17 in such a manner that openings 20 and 22 are in alignment. Mold 13 is then placed on block 15 so that openings 22 are also in alignment to receive ,the thermocouple which is sealed to head 17. Crucible 3, with perforate bottom in place, is positioned on the casting chamber and the crude uranium ingots 1 placed inside the crucible. Lid 4 and cap 8 `are placed in position and insulation 23 and 35 laid up around and `over the apparatus. Shield 24 is then properly positioned over the insulation and securely sealed ot flanges 25. The assembled unit isY evacuated by evacuating means attached to projection 21 and the apparatus is tested for leakage of air into the same.

In carrying out the process, the induction coil 28 is lowered around the shield 24 with the bottom portion of the coil slightly below the bottom of crucible 3 as shown in Fig. l. The coil is operated at sufficient power' to bring the temperature of the uranium to approximately 1300 C. which temperature is maintained until the molten uranium flows out of the melted ingot 1,

through orifice 12, `and into casting chamber 14. The I coil is then lowered to the position indicated by dotted lines in Fig. l in which the lowerend of the coil is 4 y approximately level with the bottom of casting chamber 14. A temperature of approximately 1300 C. is maintained for from iive to fifteen minutes in the casting chamber to vaporize the volatile impurities remaining with the cast uranium. Where the ingots used are the result of the reduction of a uranium Ihalide with magnesium, the above step is particularly important in that it removes substantially all of the magnesium associated with the cast uranium. The power to the coil 28 is f then shut off and the apparatus permitted to cool. After cooling, the vacuum isreleased and the apparatus disassembled for removal of the puried uranium. j t In melting the uranium in chamber 2, it is preferred that the heating be carried out at such a rate that the temperature ofV the. uranium'throughout the'ingo't does not appreciably lag behind the temperature of the uranium adjacent the walls of crucible 3, or the temperature of the carbon walls. Where an induction coil is used to heat the crucible, the temperature of the uranium in the crucible is controlled by increasing or decreasing the power supply to the induction coil. Thus, Where the rate at which the crucible and its contents are being heated is;, rapid, the power supplyl to the induction coil is decreased. where the rate at which they crucible and its contents are being heated is not rapid enough, more power is supplied to the induction coil. In this way, uranium carbidey formation in chamber 3 is reduced and violent ebullition of the volatile impurities is prevented during the melting operation. Such uniform heating is particularly desir able in the initial stage of melting the uranium. A slightr lag between the temperature of the inner and outerpor-A tions of the ingot is not objectionable `although the tem` perature of the outer portion should not be permitted to rise to a point where there is excessive `formation of: uranium carbide. p

It has been found that pipes or hollows in the upper',l surface of the final casting may :be prevented and bet-n ter castings generally obtained by regulating the cooling` of the cast metal in such a manner that the solidiication begins at the bottom of the casting chamber andproceeds upwardly. This may be done either by having the upper portion of mold 13 at a higher temperature thank the lower portion when the casting is made, or by gradfAv ually raising coil 28 from the dotted line position of; Fig. 1 after the uranium has owed into chamber 14 to'VV maintain the upper portion of the casting chamber at -af higher temperature than the lower portion. Preferably, some heating is supplied to the upper portion of the'.- casting chamber while the cast uranium is cooling. Y The withdrawal of the molten uranium from the impurities through orifice 12 and into chamber 14 during the casting operation results from the weight ofthe molten uranium 30 bearing upon the portions of'lm 29 between vsupporting points 31, as shown in Fig. 4. As' the uranium is converted to a molten state with film 2 Y being exible but relatively inelastic, the molten uranium tends to stress the film :between supporting points 31, that is into orifices 12. Eventually, theskin bursts the molten uranium ows through orifices 12 and intr)- chamber 14, leaving film 29 in melting chamber 2. It is contemplated that means of various shapes and sizes` may be used to provide supporting points 31. f

Preferably, the inner surface of melting, chamber 2i and particularlythe surface of sloping sides 10 have a i hereable characteristics which aid in maintaining the skin in chamber 2. It has been found that the skin 29 tends to adhere to machined graphite as the molten uraniumKV ows through orifices 12.l Where a plurality of ingots 211'#v treated, .the side wallsA of chamber 2 retainy portions lof the skin of the top ingots. If desired, a perforated bottom 5 defining slots 32 instead of holes 12, as shown in Fig. 3, may be used.` Such a modification is used to facilitate bursting of s 29 during, the castingoperation. This results ,from increased areabetween supportingpoints ,31.

`Where ingots 1 Ahave unusually tough skins, a ring 33 'as shownin Fig. 5, may be placed over vperforate bottom 5 to support the ingots. Ring 33 may be formed of graphite and has side openings to pass the molten uranium to bottom 5. Ring 33 is used `to give a very large area .between the supporting points which in this instance will be the upper edges of the ring.

The casting process may be carried out in the presence of air at atmospheric pressure, although there is a greater tendency for the formation of uranium oxide on the cast uranium than where the process is carried out under vacuum or in an inert atmosphere. In a specific example of casting the uranium in the presence of air, a uranium ingot weighing 8.37 pounds was placed in the melting chamber and the apparatus assembled as shown in Fig. 1. The inductio-n coils were used to raise the temperature of the ingot to l400 C. in ten minutes. This temperature was held for five minutes and then lowered vto approximately 1165 C. and the latter temperature maintained Afor fifteen minutes, giving a total heating period of thirty minutes. The molten uranium broke through the film of impurities and flowed into the casting chamber. The apparatus was allowed to cool for several hours and then disassembled. The impurities which remained in the melting chamber were flaky and comprised approximately percent of the total weight of the ingot. The cast uranium was removed from the casting chamber and was comparable in appearance to the recasting carried out in vacuo. After being sandblasted, the ingot was found to weigh 7.25 pounds, thus giving a casting yield of approximately 87 percent.

In heating ingots resulting from the reaction of a uranium halide and a substantial excess of reducing metal such as calcium or magnesium, there is a rapid vaporiztv tion of the reducing metal before the uranium metal ows into the casting chamber. This sudden vaporization, resembling an explosion, usually takes place at approximately 1050" C. where magnesium is used, and at between 130()o C. and 1400 C. where calcium is used. It has been -found that such violent vaporization may be prevented by decreasing the mount of reducing metal used during the reaction to a point where there is only sufficient excess to obtain a complete reduction of the uranium halide.

Where the melting chamber 2 and casting chamber iff-i are formed of carbon such as graphite, it is desirable to keep the temperature of the uranium in such containers below from 1600 C. to 1700 C. to prevent excessive formation of uranium carbide. Uranium carbide is not formed to any great extent where the molten uranium is cast below l600 C. if the time of contact between the molten uranium and the carbon is kept to a minimum, for example, 10 to 2G minutes. It has been found that such formation increases rapidly where the temperature of molten uranium is above from 1600 C. to l700 C. when cast. Where it is desired to obtain the cast uranium metal with a minimum of titanium carbide, the step of heating mold 13 with coil 28 in its lower position to vaporize the volatile impurities after the uranium has iiowed into chamber 14 may be omitted.

The term uranium ingot, as used in the description and claims includes any metallic uranium mass containing impurities of appreciable amount such as skin fragments which are soiid at the melting point of uranium. Thus, it has been used to define a crude uranium ingot which contains impurities resulting from the production of the uranium, as well as any mass of uranium metal which has been exposed to air, thereby having uranium oxide formation on the outer surface of the mass.

The above detailed description is for purposes of illustration and the invention is to be limited only by the scope of the following claims.

What is claimed is:

l. A process of treating a uranium ingot having an inner uranium metal portion and an outer oxide skin portion which comprises partially supporting saidfingot at spaced areas of its bottom surface, providing for a suiicient weight of uranium so that it causes bursting of the oxide skin when molten, uniformly heating said ingot while in Contact with graphite surfaces to the melting point of the uranium thereby bursting said skin portion at the unsupported bottom surface whereby the molten uranium flows through said bursted skin while retaining said skin in its original position, and separating said molten uranium by gravity substantially perpendicularly to said bottom surface.

2. A process of treating a uranium ingot having an inner uranium metal portion and an outer oxide skin portion which comprises partially supporting said ingot at spaced areas of its bottom surface, providing for a sufficient weight of uranium so that it causes bursting of the oxide skin when molten, heating said ingot Within ten minutes to approximately 1400 C., maintaining said temperature for about live minutes, reducing said temperature to substantially 1165o C. and maintaining said latter temperature for about fifteen minutes thereby bursting said skin portion at the unsupported bottom surface and owing the molten uranium through said bursted skin while retaining said skin in its original position.

3. A process of treating a uranium ingot having an inner uranium metal portion and an outer voxide skin portion which comprises partially supporting said ingot at spaced areas of its bottom surface, providing for a sufficient weight of uranium so that it causes bursting of the oxide skin when molten, heating said ingot while in contact with graphite surfaces to at least the melting point of the uranium but below 1600 C. thereby bursting said skin portion at the unsupported bottom surface and fiowing the molten uranium through said bursted skin while retaining said skin in its original position.

4. A process of treating a uranium ingot having an inner uranium metal portion and an outer oxide skin portion which comprises partially supporting said ingot at spaced areas of its bottom surface, providing for a suflicient weight of uranium so `that it causes bursting of the oxide skin when molten, heating said ingot while in contact with graphite surfaces to the melting point of uranium but below 1600 C. in a substantially oxygenfree atmosphere thereby bursting said skin portion at the unsupported bottom surface whereby the molten uranium iiows through said bursted skin while retaining said skin in its original position, yand separating said molten uranium by gravity substantially perpendicularly to said bottom surface.

5. A process of treating a uranium ingot having an inner uranium metal portion and an outer oxide skin portion which comprises partially supporting said ingot at spaced areas of its bottom surface, providing for a sufficient weight of uranium so that it causes bursting of the oxide skin when molten, heating said ingot while in contact with graphite surfaces under reduced pressure to the melting point but below 1600" C. thereby bursting said skin portion at the unsupported bottom surface whereby the molten uranium Hows through said bursted skin while retaining said skin in its original position, and separating said molten uranium by gravity substantially perpendicularly to said bottom surface.

6. A process for purifying a uranium ingot having an inner uranium metal portion and an outer oxide skin portion which comprises partially supporting said ingot at spaced areas of its bottom surface, providing for a sufficient weight of uranium so that it causes bursting of the oxide skin when molten, heating said ingot while in contact with graphite surfaces to the melting point of the uranium but below 1600 C. thereby bursting said skin portion at the unsupported bottom surface whereby the molten uranium ows through said bursted skin while said skin is retained in its original position, separating p '7 8 4said molten uranium by gravity substantially perpendicu- 1,648,954 Marden Nov. 15,` 1927 larly to said bottom surface, allowing said separated 1,648,962 Rentschler et al. Nov. 15, 19,27 uraniumrnetal gradually to solidify by starting to reduce 1,657,952 Zoda Ian. 31, 1928 fr," the temperatureat the bottom layer of the purified 1,839,196 Loth Dec. 29,1931",v i uranium and gradually proceeding upwardly with cooling. 5 2,085,450 Rohn ,Iune 29, 1937` 2,133,634 Rohn ...4 Oct., 18, 1938 `llefel'ellces in the fue Of this patent v v A UNITED STATES PATENTS Moore: Preparation of Metallic Uranium, Transactions 4, 1,292,582 Coulson Ian. 28, 1919 10 of the American Electrochemical Society, vol. 43, pp. `-f

1,568,685 Moore Jan. 5, 1926 323, 327 (1927). 

1. A PROCESS OF TREATING A URANIUM INGOT HAVING AN INNER URANIUM METAL PORTION AND AN OUTER OXIDE SKIN PORTION WHICH COMPRISES PARTIALLY SUPPORTING SAID INGOT AT SPACED AREAS OF ITS BOTTOM SURFACE, PROVIDING FOR A SUFFICIENT WEIGHT OF URANIUM SO THAT IT CAUSES BURSTING OF THE OXIDE SKIN WHEN MOLTEN, UNIFORMLY HEATING SAID INGOT WHILE IN CONTACT WITH GRAPHITE SURFACES TO THE MELTING POINT OF THE URANIUM THEREBY BURSTING SAID SKIN PORTION AT THE UNSUPPORTED BOTTOM SURFACE WHEREBY THE MOLTEN URANIUM FLOWS THROUGH SAID BURSTED SKIN WHILE RETAINING SAID SKIN IN ITS ORIGINAL POSITION, AND SEPARATING 